Four wheeled vehicles may have a driveline configuration allowing the drive train torque to be distributed longitudinally for enhancing lateral vehicle dynamics or for other enhancements, such as traction performance, noise reduction, fuel consumption, etc.
One of the primary objectives with controlling the longitudinal torque distribution in a motor vehicle is to change the effective yaw moment for vehicle dynamics reasons. A bicycle model with tire forces is shown in FIG. 1a-c. In FIG. 1a, showing the tire forces and the net yaw moment in the asymmetric plane of the vehicle, the vehicle is subject to an effective yaw moment 1. Here, the tractive and braking tire forces are excluded. Lateral tire forces 2, 3 are not only affected by the slip angles 4, 5, but due to the tire force ellipse 6 shown in FIG. 1b also by the longitudinal tire forces 7, 8 indicated in FIG. 1c, showing the tire forces and the net yaw moment in the asymmetric plane of the vehicle including the tractive and braking tire forces. This results in changed lateral tire forces 9, 10 and effective yaw moment 11.
This phenomenon is well known and widely used for attribute tuning of the yaw response of the vehicle. However, the same phenomenon also leads to trade-offs and compromises when controlling the longitudinal torque distribution for any other reason than lateral vehicle dynamics, such as e.g. fuel optimization, traction enhancement, noise, durability etc.
Vehicles having a capability of changing the driveline configuration may use torque vectoring. For example, a device for vectoring torque is described in WO2010/101506 by the same applicant.
One of the primary objectives with vectoring drive train torque in a motor vehicle is to change the effective yaw moment acting on the vehicle.
In FIGS. 2a and 2b, models of a vehicle having torque vectoring capabilities are shown. In FIG. 2a, the effective yaw moment 12 of a vehicle having lateral torque vectoring with front biased longitudinal torque distribution is shown. In FIG. 2b, the effective yaw moment 13 of a vehicle having lateral torque vectoring with rear biased longitudinal torque distribution is shown. In FIGS. 2a and 2b, the effective yaw moment 12, 13 is the sum of a net yaw moment 14, 15 excluding torque vectoring, and a second net yaw moment 16, 17 induced solely from lateral torque vectoring.
The net yaw moment 16, 17 induced from lateral torque vectoring 18 is not only a product of the differentiated longitudinal tire forces 19 and vehicle geometry, but is also an effect of altered lateral tire forces when the working points 20, 21 for the combined longitudinal and lateral tire forces change along the tire force ellipse. This phenomenon complicates the control of a lateral torque vectoring device as the relation between induced yaw moment 16, 17 and added torque input 18 becomes dependent on initial longitudinal tire forces. That is although the superimposed longitudinal tire forces 19 induced from the torque vectoring device are the same in FIGS. 2a and 2b, their effect 22, 23 on the total lateral force of wheel pairs are dependent on the initial longitudinal tire forces from wheel pairs propulsion plant.
As a consequence of the above described relations between longitudinal and lateral tire forces and their effect on the yaw moments 12, 13, 14, 15, 16, 17, it is troublesome to achieve consistent lateral vehicle behavior when distributing torque sideways in a vehicle with a non-fixed longitudinal torque distribution. This is due to the fact that both longitudinal and lateral torque redistribution affects the net yaw moment, and the yaw control authority of longitudinal and lateral torque redistribution, respectively, are dependent on the actual state of one another.